The invention relates generally to a freezer apparatus for rapidly freezing food products and, more particularly, to a food freezer apparatus using a combination of mechanical refrigeration and a liquid cryogen such as nitrogen for cooling food products within the freezer apparatus.
As the consumer demand for pre-prepared food products has steadily increased, the food industry has sought to develop better processes, techniques and equipment for preparing such foods and preserving them in a manner that will maintain the character of the foods and permit them to be reconsituted to an acceptable form for consumption. The preferred method of preserving such pre-pared foods is through quick freezing of the prepared foods in such a manner that there is little or no deterioration in food product. Since the process of freezing foods tends to dry or desiccate the foods, and since the drying rate is a function of the freezing conditions, it is important that the freeing process be controlled in a manner that will minimize the drying of the food.
The problem of freezing prepared food products is further complicated by the fact that the pre-prepared foods often arrive at the freezing process at a very elevated temperature. As a consequence, there is a greater temperature span through which the food must be cooled before it reaches the frozen condition at which no further drying will occur. Experience has indicated that the most satisfactory solution to the quick freezing of food products to minimize moisture loss has been through the use of liquid cryogen, which provides substantially lower temperatures than are available with conventional mechanical refrigeration. Although mechanical refrigeration freezers have been designed to produce temperatures below -40.degree. F. for food freezer application, there is another problem with mechanical refrigeration freezers that makes their use for fast freezing precooked foods undesirable. As a consequence of the high vapor pressure of water at temperatures around 100.degree. F. and above, dehydration in hot, precooked products entering a freezer does not take place as a steady state transfer but as miniature explosions in the food's surface. The low temperature air produced by mechanical refrigeration is extremely dry which has a tendency to create those explosive evaporation conditions, thereby dehydrating the food being frozen by this process.
Use of liquid cryogenic cooling in food freezers is well known in the art. Examples of patents directed to cryogenic food freezers are U.S. Pat. No. 4,953,365 to Lang et al. and U.S. Pat. No. 4,739,623 to Tyree Jr. et al. Although the liquid cryogen freezer has the advantage of being able to freeze a food product quickly in order to minimize any deterioration through moisture loss, the liquid cryogen freezer is significantly more expensive to operate than a mechanical refrigeration freezer in freezing the same quantity of food. This economic disadvantage inherent with the completely cryogenic freezer has been recognized, and there have been attempts to combine cryogenic and mechanical refrigeration to achieve the quick freezing advantages of liquid cryogen while minimizing cost disadvantages addition. The patent to Rasovich, U.S. Pat. No. 4,858,445, is an example of a combination cryogenic and mechanical freezing system. In addition, the patent to Tyree Jr. et al., U.S. No. 4,356,707, shows various embodiments, at least one of which contemplates the use of mechanical refrigeration in combination with a liquid cryogen.
In order for the combination freezer to be most effective and economical, the cryogen liquid must be used in the initial stage to freeze the outer surface of the food product quickly, to prevent or limit moisture loss and to rely on mechanical refrigeration to continue freezing the central portion of the food product. Because of the tendency of the heated core of the product to thaw the frozen exterior, it is necessary that the freezing be continuous with no interruption which might permit the surface thawing. Studies have shown that the ability of typical foods to move refrigeration away from the food surface dramatically decreases as freezing progresses. Thus, while the effectiveness of the freezing process can decrease as food progresses through the freezer, it is necessary to maintain the frozen surface to avoid further moisture loss. As a consequence of this lessening of freezing effectiveness, the amount of cooling required to lower the exterior surface to freezing from an elevated temperature and to freeze the exterior surface may be substantially equal to the amount of cooling required to completely freeze the interior of the food product. Due to the decreasing efficiency of the freezing process, it is difficult to maintain the surface in a frozen condition while removing the heat from the interior of the food product.
To illustrate the cost savings that are possible in using a process that combines optimum amounts of mechanical refrigeration cooling along with cryogenic cooling, a typical commercial example of freezing fully cooked, formed chicken breast patties will be described in terms of required heat input. Actual measured data indicates that the heat required to reduce the temperature of patties entering the freezer at 174.degree. F. to 32.degree. F. is 127 BTU/lb. Once the food product, chicken patties in this example, is lowered in temperature to the frozen or near frozen state, food dehydration virtually ceases. In order to complete the freezing, the temperature of the patties must be reduced to 0.degree. F. which requires an additional 116 BTU/1b. Thus, this data illustrates that the portion of the freezing cycle in which moisture loss is the most prevalent, i.e. above 32.degree. F., requires slightly more than one-half of the total heat input.
Since it is important to minimize the moisture loss to maintain the quality of the food being frozen, it is desirable to use the cryogen cooling during the initial stage in which the temperature is reduced to freezing or near freezing while using mechanical refrigeration for all the cooling thereafter. The principal obstacle to this approach is the tendency of the higher temperature core of the food product to thaw the frozen surface as the food product proceeds through the freezer. To accomplish the objective of providing a freezer apparatus using a combination of cryogenic cooling and mechanical refrigeration, the two types of cooling must be combined to provide uninterrupted, continuous cooling of the food products.
Commercial food freezers may be broadly categorized as batch type; in which successive loads are processed through the freezer, or continuous type freezers in which products move continuously through the freezing zone or zones on some type of conveyor belt. The present invention involves a continuous freezer apparatus which includes a spiral or helical-type product conveying system. The Tyree Jr. et al. U.S. Pat. No. 4,356,707 discussed above in connection with the combination of mechanical refrigeration and liquid cryogen also shows the use of a spiral conveyor for continuously transporting the food articles through the freezer apparatus. Other examples of patents showing spiral conveyors used in cryogenic freezers are Tyree Jr. et al. U.S. Pat. No. 4,739,623 and Sink et al. U.S. Pat. No. 4,947,654. The U.S. Pat. No. 3,412,476 to Astrom shows the use of a helical conveyor in combination with a mechanical refrigeration apparatus using cylindrical baffles to direct circulating cooling air through the conveyor.
The spiral conveyor is recognized as an efficient and space-saving type of conveyor to use in a freezer apparatus. The typical spiral conveyor has a series of loops or turns spiraling around a rotating core which serves to drive the conveyor. A straight entrance portion conducts the products into the spiral and a similar straight exit portion delivers the frozen products through an exit opening in the insulated enclosure. By providing many linear feet of conveyor travel in a relatively small amount of floor area through the use of the multiple tiered or superimposed loops, the spiral conveyor is more attractive from a space-saving standpoint than many types of straight-line conveyors which require elongated tunnels in which the food processing or freezing would take place.